Conventionally, a compressor used in a motor vehicle air conditioner includes an electromagnetic clutch provided between the air conditioner and a driving power source for transmitting power.
An electromagnetic clutch can be used to choose the transmission and non-transmission of power with an electromagnetic force, and, as shown in FIG. 19 for example, it is configured such that an armature 2 is attracted toward a rotor 3 by magnetic force caused by an electromagnetic coil 1 so as to couple the armature 2 with the rotor 3 for transmitting power (See Patent Literature 1, for example.). The exemplary configuration as shown in the drawing, the widths of the armature 2 and the rotor 3 in their radial direction are segmented into two and three, respectively, so that the contact surface (gap) 4 between them are segmented into four in the radial direction. Hereinafter, the contact surface 4 on the side of the rotor 3 is called an armature contact surface 4a and the contact surface 4 on the side of the armature 2 is called a rotor contact surface 4b. 
As shown in FIG. 20A for example, the armature contact surface 4a of the rotor 3 is segmented into three in the radial direction by two grooves 5 having a groove width of a to form an inner ring 3a, an intermediate ring 3b, and an outer ring 3c. The two grooves 5 are circumferentially split at a plurality of portions by bridges 6 bridging the inner ring 3a, the intermediate ring 3b, and the outer ring 3c. 
As shown in FIG. 20B for example, the contact surface of the armature 2 is also segmented into two in the radial direction by a groove 7 having a groove width of b to form an inner portion 2a and an outer portion 2b. The groove 7 on the side of the armature 2 is circumferentially split into several portions by bridges 8 bridging the inner portion 2a and the outer portion 2b. In this case, the armature 2 is manufactured from a plate-like material by punching.
As is well known in the art, the grooves 5 and 7 are formed to obstruct magnetic flux or magnetism, and called a magnetic gap.